Pyrolysis-based fuel processing method and apparatus

ABSTRACT

The method for generating a hydrogen-rich stream from hydrocarbon fuels, ultimately to produce hydrogen gas, involves the following two steps performed in a cyclic fashion: (1) pyrolysis of the hydrocarbon fuel to obtain a carbon-rich fraction and a hydrogen-rich fraction; and (2) oxidation of the carbon-rich fraction, or a portion of it, for heat generation. The method involves the following optional steps: (3) steam gasification of part of the carbon-rich fraction to produce additional amounts of hydrogen and carbon monoxide; (4) water-gas shift reaction to convert carbon monoxide to carbon dioxide with the simultaneous formation of additional amounts of hydrogen; and (5) steam reforming of light hydrocarbons that may be produced in step (1) to produce more hydrogen and carbon monoxide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/216,888, filed Jul. 7, 2000, in the names of the inventors designatedherein and bearing the same title.

STATEMENT REGARDING GOVERNMENT INTEREST

The United States Government has rights in this invention under NationalScience Foundation grant No. DMI-9632781.

BACKGROUND OF THE INVENTION

Fuel cells which are currently of commercial interest operate on streamsof pure or nearly pure hydrogen, which is not readily available in mostvehicles. Neither is a source of pure hydrogen convenient or safe tocarry on board commercial trucks, buses or other vehicles. However,liquid hydrocarbons, such as diesel fuels, are easily available andtheir handling, storage and distribution are well developed.Consequently, the large-scale use of fuel cells is expected to requireconversion of liquid fuels into a stream of pure hydrogen orhydrogen/CO₂ mixtures, with only trace amounts of CO or sulfurimpurities. This conversion will require a multi-step process to becarried out on board vehicles.

Dramatic progress has been observed in fuel-cell technologies in recentyears. A prototype fuel cell-powered bus has been built by Ballard PowerSystems for Vancouver's BC Transit. In this bus, compressed hydrogen isused to fuel the cells, which has raised concerns about passengers'safety. In a different venture, Argonne National Laboratory has builtthree prototype buses running on fuel cells. These vehicles operate withthe diesel engine replaced by an electric engine, a phosphoric-acid fuelcell, and an on-board reformer. The role of the reformer is to convertliquid methanol into hydrogen in situ, and thus to avoid the necessityof carrying pressurized hydrogen. It is interesting to note thatArgonne's fuel cell and the reformer are not much larger than the dieselengine they replaced. The fact that methanol is not currently a widelyused fuel poses obvious limitations. There are also concerns related tolong-term viability as well as corrosiveness and toxicity of methanol.

The development of an on-board system capable of converting hydrocarbonfuels, such as gasoline, diesel, JP-5, natural gas, etc., into a streamof hydrogen-rich gas would make it possible to power vehicles usingstandard fuels in combination with fuel cells. This would greatlyaccelerate the introduction of fuel ell technologies into mass transitand help reduce air pollution in urban centers (particulates, NO_(x),CO, and unburned hydrocarbons). The advantage of on-board fuelprocessing is clear: the utilization of conventional fuels at improvedefficiency, lower pollution levels, and zero noise.

-   Partial Oxidation—One current approach to the conversion of standard    liquid fuels into hydrogen is partial oxidation (POX) of the liquids    to produce soot, carbon oxides and hydrogen. The reaction is    normally carried out without a catalyst in the temperature range    1100-1500° C. This technology is similar to the process used in the    manufacture of carbon black (Austin, G. T., Shreve's Chemical    Process Industries, Fifth edition, McGraw-Hill, New York, 1984). A    number of projects are currently under way in which fuel processors    based on partial oxidation are being developed (Preprints of the    Annual Automotive Technology Development Contractors' Coordination    Meeting, PNGV Workshop on Fuel Processing for Proton Exchange    Membrane (PEM) Fuel Cells, Dearborn, Mich., 23-27 Oct., 1995, Office    of Transportation Technologies, U.S. Department of Energy,    Washington, D.C., 1995; Preprints of the Anal Automotive Technology    Development Contractors' Coordination Meeting, vol. I, Dearborn,    Mich., 23-27 Oct., 1995; “Recent Advances in Fuel Cells,” M. A.    Wójtowicz, Symposium Organizer, in ACS Div. of Fuel Chemistry Prepr.    44 (4), pp. 972-997, 1999; “Hydrogen Production, Storage, and    Utilization,” C. E. Gregoire-Padro and F. S. Lau, Symposium    Organizers, in ACS Div. of Fuel Chemistry Prepr. 44 (4), pp.    841-971, 1999). The advantages of partial oxidation include    simplicity, exothermicity of the process, sulfur tolerance, rapid    start-up, rapid response to load changes, and compactness. However,    partial oxidation produces relatively small amounts of gaseous    hydrogen, which is diluted with nitrogen, large amounts of carbon    oxides and soot, and the efficiency of fuel utilization is    relatively low.-   Steam Reforming—A second approach is based on steam reforming of    hydrocarbon fuels according to the following reaction:    C_(n)H_(m)+n H₂O→n CO+(n+m/2)H₂  (A)-   wherein n and m are typically in the range 1-20 and 4-42,    respectively.

Since the above reaction is endothermic, the unreacted hydrogen from thefuel cell is usually burned to provide process heat. The reaction occursover a catalyst in the temperature range 700-1000° C.

Since proton-exchange membrane (PEM) fuel cells, which are typicallyused in transportation applications, are intolerant to carbon monoxide,the latter species present in the product gas is often shifted to carbondioxide according to the following reaction:CO+H₂O<→CO₂+H₂  (B)

Shift conversion is usually carried out in two stages: ahigh-temperature stage followed by a low-temperature stage. The formerstage promotes high reaction rates, whereas the low-temperature stageincreases the yield. Since the water-gas shift reaction is exothermic,inter-stage cooling is often implemented. In high-temperature fuelcells, CO can be oxidized to CO₂ directly, and no shift reaction isnecessary.

Steam reforming is a well-established large scale technology, butdesign, construction, and operation of compact reformers is quite achallenge. Common feedstocks for steam reforming are natural gas,propane and butane. The use of heavier feedstocks, such as naphtha, isdifficult, and this problem can be only partly alleviated by the use ofspecially prepared catalysts (Austin, G. T., Shreve's Chemical ProcessIndustries, Fifth edition, McGraw-Hill, New York, 1984). In most cases,a desulfurization step is required upstream of the reformer to protectcatalyst beds from deactivation.

-   Autothermal Reforming—Autothermal reforming (ATR) is a hybrid    approach involving endothermic steam reforming combined with partial    oxidation for heat generation. The fuel is mixed with a mixture of    steam and air, preheated, and fed into a catalytic reactor. Proper    control of the steam-to-fuel ration is required to avoid coke    formation, and the reaction usually occurs at 650-700° C. The    effluent is typically sent to a shift reactor prior to entering the    fuel cell. The advantages of ATR include compactness, and nitrogen    dilution is the main disadvantage. The efficiency of ATR is lower    than that of steam reforming but higher than POX.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram providing an overview of a reaction scheme embodyingthe present invention.

FIG. 2 is a flow diagram of a system embodying the invention, comprisinga diesel processor, a fuel cell, and auxiliary components (steamreformer, shift reactor, sulfur and CO removal units, etc.).

SUMMARY OF THE INVENTION

It is a broad object of the present invention to provide a novel methodfor producing a stream of hydrogen-rich gas, and thereby for producinghydrogen gas, from a hydrocarbonaceous material.

It is also an object of the invention to provide a power system whereinhydrogen gas for use in a fuel cell is produced from a hydrocarbonaceousmaterial, and wherein the system may be self-contained and implementedin a transport vehicle.

It has now been found that certain of the foregoing and related objectsof the invention attained by the provision of a method for producinghydrogen gas from a hydrocarbonaceous material, using reaction apparatusthat includes means for absorbing and releasing thermal energy andhaving a heat-transfer surface. The method comprises the followingsteps, carried out cyclically:

(a) bringing a quantity of a hydrocarbonaceous material into contactwith the surface of the means for absorbing and releasing thermalenergy, heated to a temperature T_(max), to effect pyrolysis thereof andthereby to produce quantities of solid carbon-rich residue and hydrogengas;

(b) effecting combustion of at least a first portion of the quantity ofthe carbon-rich residue produced in the pyrolysis step; and

(c) utilizing at least a portion of the thermal energy produced in thecombustion step to heat the means for absorbing and releasing thermalenergy to T_(max), for effecting the pyrolysis step in the nextsucceeding cycle of the method.

The method will preferably include the additional step of (d) effectingsteam gasification of a second portion of the solid carbon-rich residueproduced in the pyrolysis step and deposited on the heat transfersurface. In accordance therewith, steam may be introduced into thereaction apparatus subsequent to the pyrolysis step, for reaction withthe second portion of the carbon-rich residue to effect the steamgasification step, with the sensible heat of the means for absorbing andreleasing thermal energy supplying the heat necessary; the portion ofthermal energy produced in the combustion step and used for heating themeans for absorbing and releasing thermal energy would, in suchinstances, be sufficient to supply the energy necessary for both thepyrolysis step and also the steam gasification step.

In most embodiments of the method a quantity of carbon monoxide isproduced, directly or indirectly, from the hydrocarbonaceous material,and the method desirably includes the additional step of (e) effecting awater-gas shift reaction, utilizing at least a portion of the quantityof carbon monoxide produced, so as to produce carbon dioxide and anadditional quantity of hydrogen gas. The method may also include theadditional step of (f) effecting steam reforming of gaseous hydrocarbonsproduced in the pyrolysis step, preferably using thermal energy producedin the combustion step. The means for absorbing and releasing thermalenergy may comprise a bed of a catalyst that is effective for promotingpyrolysis of the hydrocarbonaceous material.

Other objects of the invention are attained by the provision of a powersystem comprising a fuel cell, which utilizes hydrogen for powergeneration, and reaction apparatus for producing hydrogen gas,operatively connected for delivering hydrogen gas produced thereby tothe fuel cell. The reaction apparatus employed will include: means forabsorbing and releasing thermal energy and having a heat transfersurface; means for introducing a hydrocarbonaceous material into theapparatus and for depositing the material upon the heat transfer surfacethereof, for effecting pyrolysis of the material; and means forintroducing an oxygen-containing gas into the apparatus for effectingcombustion of carbon produced by pyrolysis of the depositedhydrocarbonaceous material, and for thereby delivering thermal energy tothe means for absorbing and releasing thermal energy.

In preferred embodiments the system will be self-contained, and willadditionally include means for storing a supply of hydrocarbonaceousmaterial, operatively connected to the means for introducing. Such asystem may be part of a transportation vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fuel-conversion process is divided into several phases thatpreferably take place in the same reactor. The reactor mass, includingpacking (which will preferably comprise a catalyst bed), is used as aheat-transfer medium in such a way that the heat required by endothermicreactions is provided from preceding exothermic cycles. Thus, thereactor mass, which may comprise the reactor walls, catalyst bed,refractory liners, any suitable packing that increases the thermalcapacity of the system, etc., constitutes the means for absorption andrelease of heat. The operation of the fuel processor is described insteps a-d below. FIG. 1 provides an overview of reaction pathways.

-   At a cold start (not included in FIG. 1), the fuel is burned with    air within the reactor volume until the maximum temperature of the    reactor mass T_(max.) is reached. The exhaust gases from this cycle    are discarded. As an alternative, a rapid start-up could be achieved    by the initial heating of the reactor with a hydrogen flame. The    hydrogen needed for start-up would be stored in a small hydrogen    reservoir.    fuel+O₂→CO₂ and H₂O (heat generation for cold start-up)  (1)    or    H₂+½ O₂→H₂O (heat generation for cold start-up)  (2)-    It should be noted that any other suitable way of warming up the    reactor can be used, which will be apparent to those skilled in the    art.    -   a) In the next step, the air supply is cut off and the fuel is        thermally or catalytically cracked (pyrolyzed) on hot reactor        surfaces to produce carbon and hydrogen as the main products.        Some quantities of light hydrocarbons (mostly methane) and a        heavy-hydrocarbon deposit are also formed. An optimal product        distribution would be carbon and hydrogen, with only small        amounts of light and heavy hydrocarbons. If the amount of light        hydrocarbons produced is small, the product gases could be        utilized by the fuel cell directly. Otherwise, a reforming step        may be desirable to convert light hydrocarbons to hydrogen.        fuel→H₂+C+C⁻⁴+heavy HC's (fuel pyrolysis)  (3)        C⁻⁴+H₂O→H₂+CO (reforming)  (4)-    where C⁻⁴ denotes light hydrocarbons with four or fewer carbon    atoms, and HC's stands for hydrocarbons.-    In step (b), temperature drops from T_(max) to T₁.    -   b) The next step involves endothermic steam gasification of the        carbon-rich deposit to produce CO and H₂. The heat for this        reaction is provided by the hot reactor core, or the catalyst        bed, the temperature of which drops from T_(I) to T_(min).        C+heavy HC's+H₂O→H₂+CO (steam gasification)  (5)    -    Before entering the fuel cell, the gaseous effluent may be        treated with steam in a shift reactor to produce carbon dioxide        and more hydrogen:        CO+H₂O→CO₂+H₂ (water-gas shift)  (6)    -    In principle, the water-gas-shift reaction may occur inherently        during the gasification step if excess amounts of steam are        present in the system. Through proper design, the        water-gas-shift reaction may be integrated with the gasification        step and take place within the same reactor. The water-gas shift        reaction is mildly exothermic.    -   c) In the last cycle, the reactor core temperature is raised        back to T_(max,) by burning the remaining carbon in air or        oxygen.        C+heavy HC's+O₂→CO₂+CO (combustion)  (7)    -    The exhaust gases of this cycle may be discarded, or the CO may        be shifted to CO₂ and additional hydrogen via reaction 6.

Stages (b), (c), and (d) will be referred to as pyrolysis (or fuelcracking), gasification, and oxidation (or carbon burn-out, orcombustion), respectively.

It should be appreciated that the above steps may be carried out in asingle reactor or in multiple reactors. For example, fuel pyrolysis,steam gasification, and residue combustion may take place in the samereactor, whereas the water-gas shift reaction is implemented in aseparate reactor. In certain embodiments of the invention, however (suchas to provide a self-contained installation or transport vehicle), allthe above steps will desirably (or necessarily) be integrated within asingle reactor system.

It should also be pointed out that fuel pyrolysis (reaction 3) and thecombustion of the carbon-rich residue (reaction 7) are the necessarysteps of the process, whereas the remaining steps are optional albeit,to a greater or lesser extent, preferred. In general, the inclusion ofthe gasification step, steam reforming, and water-gas shift increasesthe efficiency of the fuel processor at the expense of increased systemcomplexity. In addition, it should be noted that:

-   T_(min) is generally determined by the condition that enough    carbon-rich deposit must be left so that the temperature can be    raised again to T_(max) in the carbon burnout step d. Another    constraint on T_(min) is of a kinetic nature: if the temperature    drops too much, the steam gasification reaction becomes unacceptably    slow, and the reactor temperature has to be raised.-   The above scheme can be implemented with and without catalysts.-   The process can be used for the processing of gaseous, liquid,    solid, and mixed hydrocarbon feedstocks.-   The products of the exothermic step (combustion) can be completely    discarded, thereby reducing the load on the water-gas shift reactor.    The secondary processing can be further simplified if thorough    cracking of the fuel to carbon and hydrogen can be effected, with    only negligible amounts of light hydrocarbon gases produced. In such    a case, the steam-reforming step is unnecessary. On the other hand,    the utilization of the water-gas shift and C⁻⁴ reforming steps leads    to improved system efficiency. High flexibility of the proposed    process will be appreciated.-   The use of multiple cycles involving high temperatures is thought to    be feasible. It should be noted that the internal-combustion engine    does involve multiple strokes occurring at high frequencies, high    pressures, and at elevated temperatures. It is expected that the    engineering of the fuel processor can be readily handled by those    skilled in the art.    An example of a reaction scheme embodying the invention is shown in    FIG. 1 for the case of diesel processing at about 1100° C. At this    temperature, the main pyrolysis products are found to be hydrogen,    carbon residue (mostly carbon), and methane. Simplified reaction    stoichiometry is given below.    -   Diesel Pyrolysis: $\begin{matrix}        {{C_{n}{H_{m}\underset{\Delta}{\longrightarrow}{xH}_{2}}} + {\frac{m - x}{4}{CH}_{4}} + {\left( {n - \frac{m - x}{4}} \right)C}} & (8)        \end{matrix}$    -   Char Gasification: $\begin{matrix}        {{{p\left( {n - \frac{m - x}{4}} \right)}C} + {{p\left( {n - \frac{m - x}{4}} \right)}H_{2}\left. O\longrightarrow{p\left( {n - \frac{m - x}{4}} \right)} \right.H_{2}} + {{p\left( {n - \frac{m - x}{4}} \right)}{CO}}} & (9)        \end{matrix}$    -   Char Combustion: $\begin{matrix}        {{{q\left( {1 - p} \right)}\left( {n - \frac{m - x}{4}} \right)C} + {{q\left( {1 - p} \right)}\left( {n - \frac{m - x}{4}} \right)\left. O_{2}\longrightarrow{q\left( {1 - p} \right)} \right.\left( {n - \frac{m - x}{4}} \right){CO}_{2}}} & \left( {10a} \right) \\        {{\left( {1 - q} \right)\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right)C} + {\frac{1 - q}{2}\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right)\left. O_{2}\longrightarrow\left( {1 - q} \right) \right.\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right){CO}}} & \left( {10b} \right)        \end{matrix}$    -   Water-Gas Shift (CO from Char Gasification): $\begin{matrix}        {{{{yp}\left( {n - \frac{m - x}{4}} \right)}{CO}} + {{{yp}\left( {n - \frac{m - x}{4}} \right)}H_{2}\left. O\longrightarrow{{yp}\left( {n - \frac{m - x}{4}} \right)} \right.H_{2}} + {{{yp}\left( {n - \frac{m - x}{4}} \right)}{CO}_{2}}} & \left( {11a} \right) \\        {\left( {1 - y} \right){p\left( {n - \frac{m - x}{4}} \right)}\left. {CO}\longrightarrow\left( {1 - y} \right) \right.{p\left( {n - \frac{m - x}{4}} \right)}{CO}} & \left( {11b} \right)        \end{matrix}$    -   Methane Reforming: $\begin{matrix}        {{r\frac{m - x}{4}{CH}_{4}} + {r\frac{m - x}{4}H_{2}\left. O\longrightarrow 3 \right.r\frac{m - x}{4}H_{2}} + {r\frac{m - x}{4}{CO}}} & \left( {12a} \right) \\        {\left( {1 - r} \right)\left. {CH}_{4}\longrightarrow\left( {1 - r} \right) \right.{CH}_{4}} & \left( {12b} \right)        \end{matrix}$    -   Water-Gas Shift (CO from Methane Reforming): $\begin{matrix}        {{{zr}\frac{m - x}{4}{CO}} + {{zr}\frac{m - x}{4}H_{2}\left. O\longrightarrow{zr} \right.\frac{m - x}{4}H_{2}} + {{zr}\frac{m - x}{4}{CO}_{2}}} & \left( {13a} \right) \\        {\left( {1 - z} \right)r\frac{m - x}{4}\left. {CO}\longrightarrow\left( {1 - z} \right) \right.r\frac{m - x}{4}{CO}} & \left( {13b} \right)        \end{matrix}$    -   Water-Gas Shift (CO from Char Combustion): $\begin{matrix}        {{{s\left( {1 - q} \right)}\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right){CO}} + {{s\left( {1 - q} \right)}\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right)H_{2}\left. O\longrightarrow{s\left( {1 - q} \right)} \right.\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right)H_{2}} + {{s\left( {1 - q} \right)}\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right){CO}_{2}}} & \left( {14a} \right) \\        {\left( {1 - s} \right)\left( {1 - q} \right)\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right)\left. {CO}\longrightarrow\left( {1 - s} \right) \right.\left( {1 - q} \right)\left( {1 - p} \right)\left( {n - \frac{m - x}{4}} \right){CO}} & \left( {14b} \right)        \end{matrix}$

Notation

p—fraction of char gasified

(1−p)—fraction of char combusted

$\frac{q}{1 - q}$—CO₂/CO ratio in combustion products

y—fraction of gasification CO shifted

s—fraction of combustion CO shifted

r—fraction of CH₄ reformed

z—fraction of CO from CH₄ reforming shifted

Overall Reaction Stoichiometry

$\begin{matrix}{{C_{n}H_{m}} + {\left\{ {{\left\lbrack {{p\left( {1 + y} \right)} + {{s\left( {1 - q} \right)}\left( {1 - p} \right)}} \right\rbrack\left( {n - \frac{m - x}{4}} \right)} + {{r\left( {1 + z} \right)}\frac{m - x}{4}}} \right\} H_{2}O} + {\frac{1 + q}{2}\left( {1 - p} \right)\left( {n - \frac{m - x}{4\quad}} \right)\left. O_{2\quad}\longrightarrow\left\{ {\frac{x}{2} + {\left\lbrack {{p\left( {1 + y} \right)} + {{s\left( {1 - q} \right)}\left( {1 - p} \right)}} \right\rbrack\left( {n - \frac{m - x}{4}} \right)} + {{r\left( {3 + z} \right)}\frac{m - x}{4}}} \right\} \right.H_{2}} + {\left\lbrack {{\left( {n - \frac{m - x}{4}} \right)\left\{ {{\left( {1 - p} \right)\left\lbrack {q + {s\left( {1 - q} \right)}} \right\rbrack} + {yp}} \right\}} + {{zr}\frac{m - x}{4}}} \right\rbrack{CO}_{2}} + {\left\{ {{\left( {n - \frac{m - x}{4}} \right)\left\lbrack {{p\left( {1 - y} \right)} + {\left( {1 - q} \right)\left( {1 - p} \right)\left( {1 - s} \right)}} \right\rbrack} + {{r\left( {1 - z} \right)}\frac{m - x}{4}}} \right\}{CO}} + {\left( {1 - r} \right)\frac{m - x}{4}{CH}_{4}}} & (15)\end{matrix}$

-   Specific advantages of the system of the invention include:-   The pyrolysis step “splits” the original hydrocarbon feedstock    (e.g., diesel fuel) into a carbon-rich fraction (solid and/or heavy    liquid) and a hydrogen-rich fraction (gas). The separation of these    fractions occurs spontaneously.-   The heat-generating step involves combustion of the carbon-rich    fraction, without the loss of hydrogen. This is in contrast to the    POX and ATR reactors where carbon and hydrogen are combusted    indiscriminately. This selectivity in the heat generating step leads    to superior efficiency of the process as compared with POX and ATR    reactors.-   The stream of hydrogen-rich gas resulting from the process is either    undiluted or the level of nitrogen dilution is lower than in the    case of POX and ATR.-   The steam-gasification of the carbon residue is an additional source    of hydrogen originating from water. This source of hydrogen is not    present in POX and ATR processes.-   The water-gas shift reaction also produces additional amounts of    hydrogen from water. This source of hydrogen is present in the POX    and ATR processes.-   Yet another source of additional hydrogen is the reforming step in    which light hydrocarbons react with steam to produce hydrogen and CO    (reaction 4).

A description of experiments carried out to demonstrate the invention isgiven below. Although the invention can be used in conjunction withdiverse hydrocarbon fuels, the experiments described below wereperformed on diesel fuel, which is probably one of the most challengingfuels to process.

Four bench scale, fixed-bed reactors were designed and constructed, withdiameters ranging from 1″ to 1.5″, and with different designs of thefuel-injection assembly. The experimental system consisted of a tubularreactor, water and diesel injection section, gas manifold, and agas-analysis section. The entire system was computer controlled, whichallowed for automated, unattended operation throughout many cycles.

Each reactor was heated externally using a tube furnace, and furnacetemperature and inlet pressure were recorded on a continuous basis. Ahigh-pressure, dual-cylinder metering pump (Eldex A-30-S) was used forfuel delivery, and another metering pump was utilized for waterinjection. Computer controlled valves provided automatic switching fromdiesel to water at the end of the diesel-cracking stage. Both pumps wereequipped with by-pass loops to ensure smooth, trouble-free operation. Asmall stream of nitrogen was used to carry the liquid (diesel or water)aerosol into the reactor, and either air or oxygen was used to burnresidual carbon in the oxidation stage. The flow of gases at the reactorinlet was controlled by means of computer-interfaced solenoid valves.The flow rate of the gas effluent at the reactor outlet was measuredusing a digital volumetric flow meter (J&W Scientific model ADM 2000).For flow rates above 1 L/min, a Humonics model 730 bubble meter wasused. The latter device was equipped with an electronic bubble counter.

Gas analysis was performed using a Fourier transform infrared (FT-IR)analyzer and a gas chromatograph (GC). To establish reproducible,standard conditions for FT-IR gas analyses, a constant, low-flow slipstream (10 ml/min) was withdrawn from the effluent gas, and a digitalperistaltic pump was used for this purpose. The slip stream was dilutedwith nitrogen (1,630 ml/min) before entering a gas cell (On-LineTechnologies 20/20™ Multipass Cell maintained at 140° C.) of an FT-IRspectrometer (Bomem MB100). Water was condensed out of the effluentstream using two condensers: one for the main stream, and one for theslip stream. Concentrations of the following species were continuouslymonitored using FT-IR analysis: CO, CO₂, SO₂, CH₄, and other lighthydrocarbons.

Gas chromatographic analysis was performed on gas samples collected insampling bags. A Carle Series 400 AGC gas chromatograph was used tocarry out gas analysis (H₂, CO, CO₂, C₁-C₅, and C₆ or larger). Theinstrument was equipped with molecular sieve columns, a thermalconductivity detector (TCD) for the analysis of H₂, CO₂, CO, and lighthydrocarbon gases, and an SRI flame ionization detector (FID) for lighthydrocarbons. In addition, a HNU 421 GC was used. It was equipped with aflame-ionization detector (FID) for heavier hydrocarbons and an SRI110flame photometric detector (FPD) for sulfur analysis. A Chromosil 330column was used, and the oven temperature was 40° C.

Experiments involving three main components of the reaction scheme(diesel pyrolysis, steam gasification of the carbon-rich fraction, andcombustion of the residue) were conducted and product distributions weredetermined under different process conditions. An optimum nominalprocess temperature of 1,100° C. was used in most experiments.

More than 200 pyrolysis-gasification-combustion cycles were performed,and a typical pyrolysis gas composition was found to be 84 mol % H₂ and16 mol % CH₄. An average gas composition during gasification was foundto be 55 mol % H₂, 36 mol % CO, and 9 mol % CO₂. The above values do notinclude small quantities of nitrogen used as a carrier gas to entraindiesel and water aerosol and introduce them into the reactor. It isexpected that the need for a carrier gas will be eliminated in the finaldesign of the fuel processor.

Data collected in the above series of experiments were used to produce aflow-sheet design of a diesel-processor unit compatible with a 30 ft(30,000 lb) transit bus, as shown in FIG. 2, which included mass andheat balances. The assumptions and results are discussed below.

It is assumed that a complete carbon conversion to CO₂ takes place inthe char-combustion step, and the effluent gas (stream No. 7) isdiscarded. This means that the combustion-generated CO is entirelyconverted to CO₂ to recover the heat of reaction. This may beimplemented, for example in a catalytic or non-catalytic CO oxidizer(re-burner). An alternative arrangement, wherein the carbon monoxideresulting from char combustion is directed to the shift reactor so thatmore hydrogen could be generated, might be employed. This concept wouldhave to involve a CO—O₂ separation step, however, to prevent unreactedoxygen from mixing with the hydrogen formed in the shift reactor. Such astep would add unnecessary complexity and cost to the scheme, and theconfiguration shown in FIG. 2 is, therefore, deemed more advantageous.The CO oxidizer could have the form of a catalytic re-burner, forexample, with the heat of CO-to-CO₂ oxidation transferred either to thediesel processor directly or to one of its inlet streams (e.g., water,diesel, or inlet air pre-heater) using a heat exchanger. Furthermore,the CO oxidizer could be coupled with, or complemented by, aheat-recovery unit in which excess hydrogen from the outlet of the fuelcell is combusted. (Fuel cells usually operate under 20-25% excesshydrogen to keep the cell well purged and avoid contamination.)

In addition to the steam reformer and the shift reactor, thefuel-processing system is equipped with sulfur and carbon-monoxideremoval units to ensure adequate gas purity for the downstream units(the steam reformer, the shift reactor, and the fuel cell). Such unitsare commonly utilized in fuel-cell systems, and the design or selectionof these parts of the system is not the subject of this invention.

System response to transient changes in the feed rate and temperature isan important consideration related to start-up and part-load operation.Rapid start-up should be possible, e.g., by the initial heating of thereactor with a hydrogen flame. A small hydrogen reservoir could be usedto store hydrogen for the next cold start-up. Another option wouldinvolve the combustion of small amounts of diesel fuel for start-uppurposes. Part-load operation could also be facilitated by computercontrol of cycle characteristics, such as the amount of diesel injected,duration of pyrolysis, gasification, and combustion steps, etc. The useof energy-storage devices, such as flywheels, batteries, orultracapacitors, is also a possibility.

The basis for the mass-balance computations was a flow of 2,050 molH₂/hr, which is an approximate nominal hydrogen demand of a 30,000 lbtransit bus powered with a fuel cell (Fisher, J., “Fuel cell-poweredtransit bus development,” Preprints of the Annual Automotive TechnologyDevelopment Contractors' Coordination Meeting, vol. I, Dearborn, Mich.23-27 Oct., 1995). Additional assumptions upon which the mass and energybalance computations were performed are listed below.

-   In the steam-gasification step, a steam-to-carbon ratio of 3.0 (g    H₂O/g C)=2.0 (mol H₂O/mol C) was assumed based on the literature    data for steam-gasification processes (Dainton, A. D., “Gasification    of Coal,” Ch. 7 in Coal and Modern Coal Processing: An Introduction,    Pitt, G. J. and Millward, G. R., Eds., Academic Press, London, 1979,    pp. 133-162; and Van Fredersdorff, C. G. and Elliott, M. A., “Coal    Gasification,” Ch. 20 in Chemistry of Coal Utilization,    Supplementary Volume, H. H. Lowry, Ed., John Wiley & Sons, New York,    1963, pp. 892-1022).-   In the methane reformer, a steam-to-methane ratio of 3.375 (g H₂O/g    CH₄)=3.0 (mol H₂O/mol CH₄) was assumed based on the literature data    for steam-reforming of methane (Tedder, J. M., Nechvatal, A., and    Jubb, A. H., Basic Organic Chemistry, Part 5: Industrial Products,    John Wiley & Sons, London, 1975).-   20% excess oxygen in the char-combustion step.-   The amount of water in the shift reactor (a sum of H₂O in streams    No. 5, 9. and 10) was assumed to be at least 1.5 times the    equilibrium value required for the desired CO conversion (80%).-   The energy required to heat reactants to reaction temperature was    unaccounted for, assuming that most of this heat could be recovered    from the products. Although some heat loss is inevitable, it should    be borne in mind that additional energy will be available from the    fuel cell (an exothermic process), and also from the combustion of    excess hydrogen exiting the fuel cell. In addition, the conversion    of CO in the shift reactor was assumed to be only 80%, which is very    conservative and characteristic of a single-stage shift reactor    (Austin, G. T., Shreve's Chemical Process Industries, Fifth edition,    McGraw-Hill, New York, 1984). The water-gas shift reaction often    proceeds nearly to its equilibrium, which is associated with    conversions close to, and sometimes in excess of, 90% rather than    the assumed 80%.-   Results of the mass and energy balance calculations are summarized    below.

The fuel requirement for the integrated system consisting of the dieselprocessor, a shift reactor, and a methane reformer was found to be about12.5 kg/hr, i.e., approximately 10.4 L/hr (2.61 gal/hr). Thiscorresponds to a hydrogen production of about 2.05 kmol H₂/hr (˜1.02 kgH₂/hr), which is appropriate for a 50 kW fuel cell. The air requirementfor the fuel-processor was found to be about 35.4 kg/hr (1.23 kmol/hr).The entire system operates with a water requirement of 22.8 kg/hr (1.27kmol/hr), i.e., 1.82 kg H₂O/kg diesel, but using water available fromthe fuel-cell exhaust can easily compensate for this deficit. If oneincludes the fuel cell in the water balance, a surplus of 14.1 kg/hr(0.781 kmol/hr) results. The fuel-processing system can be madethermally neutral, i.e., all the energy required for the process can begenerated from diesel fuel. The overall system efficiency (excluding thefuel cell) in excess of 90% was found. The efficiency is defined as aratio of the lower heating value of the hydrogen produced to the lowerheating value of diesel.

The concept was evaluated on the basis of the available data, andcomparisons with methanol reforming and partial oxidation were made. Theabove-described system was found to offer a substantial fuel-economy andoperating-cost advantage over the methanol reformer (at least a factorof two). The main advantages over partial oxidizers are a betterefficiency (93% versus 83%) and a better quality gas feedstock for fuelcell (78 mol % H₂ for the diesel processor versus 43 mol % H₂ for apartial oxidizer). The above performance data for partial oxidizers arequoted after Mitchell, W. L., Chintawar, P. S., Hagan, M., He, B.-X. andPrabhu, S. K., “Compact fuel processors for fuel cell electric vehicles(FCEVs),”ACS Div. of Fuel Chem. Prepr., 1999, 44(4), 995-997. The maindisadvantage of the pyrolysis-based diesel processing system appears tobe its relative complexity.

Thus, it can be seen that the present invention provides a novel methodfor producing hydrogen gas from a hydrocarbonaceous material. It alsoprovides a power system wherein hydrogen gas for use in a fuel cell isproduced from a hydrocarbonaceous material, and wherein the system maybe self-contained and implemented in a transport vehicle.

1. A method for producing hydrogen gas from a hydrocarbonaceousmaterial, using reaction apparatus that includes means for absorbing andreleasing thermal energy and having a heat-transfer surface, comprisingthe following steps, carried out cyclically: (a) bringing a quantity ofa hydrocarbonaceous material into contact with said heat-transfersurface of said means for absorbing and releasing thermal energy, heatedto a temperature T_(max), to effect pyrolysis thereof and thereby toproduce quantities of solid carbon-rich residue and hydrogen gas; (b)effecting combustion of at least a first portion of said quantity ofsaid carbon-rich residue produced in said pyrolysis step; and (c)utilizing at least a portion of the thermal energy produced in saidcombustion step to heat said means for absorbing and releasing thermalenergy to said temperature T_(max), for effecting said pyrolysis step inthe next succeeding cycle of said method.
 2. The method of claim 1including the additional step of (d) effecting steam gasification of asecond portion of said solid carbon-rich residue produced in saidpyrolysis step and deposited on said heat transfer surface.
 3. Themethod of claim 2 wherein, subsequent to said pyrolysis step, steam isintroduced into the reaction apparatus for reaction with said secondportion of said solid carbon to effect said steam gasification step, andwherein the sensible heat of the means for absorbing and releasingthermal energy supplies the heat necessary for said gasification step,said portion of thermal energy produced in said combustion step and usedfor heating said means for absorbing and releasing thermal energy beingsufficient to supply the energy necessary for both said pyrolysis stepand also said steam gasification step.
 4. The method of claim 1 whereina quantity of carbon monoxide is produced, directly or indirectly, fromsaid hydrocarbonaceous material, and wherein said method includes theadditional step of (e) effecting a water-gas shift reaction, utilizingat least a portion of said quantity of carbon monoxide produced, so asto produce carbon dioxide and an additional quantity of hydrogen gas. 5.The method of claim 1 including, in said next succeeding cycle, theadditional step of (f) effecting steam reforming of gaseous hydrocarbonsproduced in said pyrolysis step.
 6. The method of claim 5 whereinthermal energy produced in said combustion step supplies the energynecessary for effecting said steam reforming step.
 7. The method ofclaim 1 wherein said means for absorbing and releasing thermal energycomprises a bed of a catalyst that is effective for promoting pyrolysisof said hydrocarbonaceous material.
 8. A method for producing hydrogengas from a hydrocarbonaceous material, using reaction apparatus thatincludes means for absorbing and releasing thermal energy and having aheat-transfer surface, comprising the following steps, carried outcyclically, (a) bringing a quantity of a hydrocarbonaceous material intocontact with said heat-transfer surface of said means for absorbing andreleasing thermal energy, heated to a temperature T_(max), to effectpyrolysis thereof and thereby to produce quantities of solid carbon-richresidue and hydrogen gas; (b) effecting combustion of at least a firstportion of said quantity of said carbon-rich residue produced in saidpyrolysis step; (c) utilizing at least a portion of the thermal energyproduced in said combustion step to heat said means for absorbing andreleasing to said temperature T_(max), for effecting said pyrolysis stepin the next succeeding cycle of said method; and (d) effecting steamgasification of a second portion of said quantity of carbon-rich residueproduced in said pyrolysis step.
 9. The method of claim 8 wherein,subsequent to said pyrolysis step, steam is introduced into the reactionapparatus for reaction with said second portion of said solid carbon toeffect said steam gasification step, and wherein the sensible heat ofthe means for absorbing and releasing thermal energy supplies the heatnecessary for said gasification step, said portion of thermal energyproduced in said combustion step and used for heating said means forabsorbing and releasing thermal energy being sufficient to supply theenergy necessary for both said pyrolysis step and also said steamgasification step.
 10. A power system comprising: a fuel cell, whichutilizes hydrogen for power generation; and reaction apparatus forproducing hydrogen gas, said reaction apparatus being operativelyconnected for delivering hydrogen gas produced thereby to said fuelcell, and including: means for absorbing and releasing thermal energyand having a heat transfer surface; means for introducing ahydrocarbonaceous material into said apparatus and for depositing thematerial upon said heat transfer surface for effecting pyrolysis of thematerial; and means for introducing an oxygen-containing gas into saidapparatus for effecting combustion of carbon produced by pyrolysis ofthe deposited hydrocarbonaceous material, and for thereby deliveringthermal energy to said means for absorbing and releasing thermal energy.11. The system of claim 10 wherein said system is self-contained. 12.The system of claim 11 additionally including means for storing a supplyof hydrocarbonaceous material operatively connected to said means forintroducing thermal energy.
 13. The system of claim 12 comprising atransportation vehicle.